Q-Chem provides access to certain singlet excited states – namely, those well-described by a single-electron HOMO-LUMO transition – via restricted open-shell Kohn-Sham (ROKS) theory. In contrast to the MOM approach (see Section 6.4), which requires separate SCF calculations of the non-aufbau and triplet energies, the ROKS approach attempts to combine the properties of both determinants at the level of the Fock matrix in one SCF calculation. ROKS thus presents as a single SCF loop, but the structure of the Fock matrix differs from the ground-state case. Note that this excited-state method is distinct from ROKS theory for open-shell ground states.

The implementation of ROKS excited states in Q-Chem largely follows the theoretical framework established by Filatov and Shaik[436] and is described in detail in Ref. Kowalczyk:2013. Singlet excited state energies and gradients are available, enabling single-point, geometry optimization and molecular dynamics.

To perform an ROKS excited state calculation, simply set the keywords ROKS = TRUE and UNRESTRICTED = FALSE. An additional keyword ROKS_LEVEL_SHIFT is included to assist in cases of convergence difficulties with a standard level-shift technique. It is recommended to perform a preliminary ground-state calculation on the system first, and then use the ground-state orbitals to construct the initial guess using SCF_GUESS = READ.

ROKS

Controls whether ROKS calculation will be performed.

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

FALSE

ROKS is not performed.

TRUE

ROKS will be performed.

RECOMMENDATION:

Set to TRUE if ROKS calculation is desired. You should also set UNRESTRICTED = FALSE

ROKS_LEVEL_SHIFT

Introduce a level shift of N/100 hartree to aid convergence.

TYPE:

INTEGER

DEFAULT:

0

OPTIONS:

0

No shift

N

level shift of N/100 hartree.

RECOMMENDATION:

Use in cases of problematic convergence.

Example 6.121 RO-PBE0/6-311+G* excited state gradient of formaldehyde, using the ground state orbitals as an initial guess.